Antimicrobial articles made from nitrile or natural rubber

ABSTRACT

Certain butadiene and/or natural rubber articles, such as acrylonitrile-butadiene rubber (NBR), natural rubber, and derivatives thereof, that exhibit highly desirable long-term effective antimicrobial characteristics. Such formulations are intended to be vulcanized to provide solid or foam rubber articles which can be utilized in a variety of different applications. Preferably such butadiene rubber formulations comprise silver-based antimicrobial compounds. As such silver-based compounds are deleteriously affected by utilization of standard non-silicone-rubber curing agents, such as sulfur-based catalysts, the ability to provide such an effective antimicrobial vulcanized rubber article is rather difficult. However, the invention encompasses the utilization of different catalyst species that permit vulcanization and silver stability for long-term antimicrobial performance of the silver-based compounds. Articles and specific vulcanizing procedures in accordance with such a novel antimicrobial vulcanized rubber are also included within the scope of this invention.

FIELD OF THE INVENTION

This invention relates to certain non-silicone vulcanized rubberarticles made from at least a majority by weight of nitrile butadienerubber and/or natural rubber that include silver-based compounds toprovide highly desirable long-term antimicrobial characteristics withinthe cured rubber articles. Such articles are in either solid or blown(foam or sponge) state (or combinations of both in multilayered forms)which can be utilized in a variety of different applications. Assilver-based compounds are deleteriously affected by utilization ofstandard curing agents and curing accelerators, such as sulfur-basedcompounds and/or systems, the ability to provide such an effectiveantimicrobial vulcanized rubber article is rather difficult. However,this invention encompasses the presence of different non-sulfur-basedcuring systems and agents, such as peroxides, as one example, thatpermit vulcanization and do not irreversibly bind silver ions thereto,thereby resulting in long-term antimicrobial performance of the ultimaterubber article itself. The rubber articles must also comprise fillersand may also include plasticizers to provide desired characteristics ofdimensional stability, stiffness, flexural modulus, tensile strength,abrasion resistance, elongation, and the like, for the ultimate rubberarticle, while simultaneously and surprisingly enhancing theantimicrobial efficacy of the rubber article as well.

DISCUSSION OF THE PRIOR ART

All U.S. Patents listed below are herein entirely incorporated byreference.

There has been a great deal of attention in recent years given to thehazards of bacterial contamination from potential everyday exposure.Noteworthy examples of such concerns include the fatal consequences offood poisoning due to certain strains of Eschericia coli being foundwithin undercooked beef in fast food restaurants; Salmonella enteritidiscontamination causing sicknesses from undercooked and unwashed poultryfood products; and illnesses and skin infections attributed toStaphylococcus aureus, Klebsiella pneumoniae, yeast (Candida albicans),and other unicellular organisms. With such an increased consumerinterest in this area, manufacturers have begun introducingantimicrobial agents within various everyday products and articles. Forinstance, certain brands of cutting boards, shoe soles, shoe inserts,medical devices and implements, liquid soaps, etc., all containantimicrobial compounds. The most popular antimicrobial for sucharticles is triclosan. Although the incorporation of such a compoundwithin liquid or certain polymeric media has been relatively simple,other substrates, specifically vulcanized rubber and surfaces thereof,have proven less accessible. Furthermore, such triclosan additives haveproven to be difficult in use or ineffective for certain bacteria. Forinstance, triclosan itself migrates easily within and out of certainpolymeric substrates and/or matrices (and thus is not very durable),lacks thermal stability, and does not provide a wide range of bacterialkill (for instance does not exhibit any kill for Pseudomonasaeruginosa).

Antimicrobial rubber formulations are certainly highly desired for theproduction of vulcanized rubber articles and compositions to provide notonly antibacterial benefits, but also antifungal, antimildew,antistaining, and odor control properties. Rubber articles are utilizedin many different applications, from automobiles (hoses, tires, bumpers,etc.), to household items (toys, sink washers, gaskets, appliances,floor mats, door mats, carpeted rubber mats, gloves, and the like), andother areas in which bacterial growth is a potential problem. There thusremains a long-felt need to provide an effective, durable, reliableantibacterial vulcanized rubber formulation which will provide suchlong-term antibacterial, etc., effects within the final vulcanizedarticle. Unfortunately, such a highly desired antimicrobial rubberformulation and/or vulcanized article containing silver-basedantimicrobial agents has heretofore not been provided by the pertinentprior art.

The closest art includes Japanese Patent Application 1997-342076 whichdiscloses the production of unvulcanized rubber formulations andarticles exhibiting antibacterial properties due to the presence ofsilver complexes. Such formulations are formed through high temperaturekneading in an oxygen-free atmosphere and are used as parts in a waterdisinfection system. Again, no vulcanized rubber is taught or obtainedwithin or through this disclosure. Antimicrobial rubber bands have beentaught in Japanese Patent Application 1997-140034 in vulcanized formwith silver antimicrobials therein. However, such compounds are ratherlimited in use and the vulcanization step must include a sulfur curingagent to effectuate the final vulcanized arrangement of the subjectrubber. Such sulfur curing agents have a remarkably deleterious effecton certain silver-based antimicrobials such that the sulfur reacts withthe silver ion to form silver sulfide, therby rendering it ineffectiveas a bactericide. As such, the utilization of such specific rubber bandformulations for and within large-scale antimicrobial articles isbasically unworkable.

Certain types of antimicrobial peroxide-catalyst vulcanized rubberformulations have been produced in the past; however, suchperoxide-cured rubbers are all silicone-based. It is well understood andaccepted that silicone rubbers cannot be vulcanized by typicalsulfur-based catalysts. Thus, the antimicrobial rubber formulations ofJapanese Patent Applications 1997-026273 and 1995-065149 as well as U.S.Pat. No. 5,466,726 are standard vulcanized silicone rubber formulationsand articles which also include certain antimicrobial compounds.

Furthermore, rubber latexes (non-vulcanized) comprising antimicrobialshave been disclosed (U.S. Pat. No. 5,736,591, for example), as havefloor mats having silver-based antimicrobials incorporated within pilefiber components and which have non-antimicrobial rubber backings curedthrough peroxide-catalyzed vulcanization to protect the pile fiberantimicrobial compounds from attack by any sulfur compounds (as inJapanese Patent Applications 1993-3555168 and 1995-38991). Again,however, to date there have been no disclosures or suggestions ofproducing a vulcanized non-silicone rubber formulation exhibitingexcellent antimicrobial properties through the long-term effectiveutilization of silver-based antibacterial compounds. This inventionfills such a void.

OBJECT OF THE INVENTION

It is therefore an object of this invention to provide an antimicrobialvulcanized rubber-containing article, wherein said rubber is selectedfrom the group consisting of nitrile butadiene rubber, natural rubber,and any mixtures thereof, wherein said article exhibits sufficientantimicrobial activity and structural integrity to withstand repeateduse without losing an appreciable level of either antimicrobial power ormodulus strength. Another object of the invention is to provide anantimicrobial vulcanized nitrile butadiene or natural rubber articlecomprising silver-based antimicrobial compounds which include curingagents which do not deleteriously effect the antimicrobial activity ofthe finished vulcanized nitrile butadiene or natural rubber article (andthus is essentially free from sulfur-based curing agents andaccelerators). Yet another object of this invention is to provide avulcanized nitrile butadiene or natural rubber-containing article thatexhibits log kill rates for Staphylococcus aureus and Klebsiellapneumoniae (and/or other types of bacteria as well) of at least 1.0after 24 hours exposure at room temperature as well as prevention ofgrowth of certain fungi after at least 15 days of exposure. Stillanother object of this invention is to provide a vulcanized nitrilebutadiene or natural rubber-containing article comprising structuralintegrity filler components and plasticizers (such as silica, metalsalts, organic salts, pigments, such as carbon black, calcium carbonate,paraffinic oils, phthalate oils, metal oxides, and the like) that alsoprovide enhancements in the control of antimicrobial efficacy of thearticle itself through regulated silver ion release to the articlesurface (e.g., exhibits higher log kill rates for Staphylococcus aureusand Klebsiella pneumoniae and prevention of growth of fungi such asAspergillus niger). Still another object of the invention is to providea finished article that exhibits increases in antimicrobial activityafter industrial washing and/or abrasion. Yet another object is toprovide a simple method of producing such an atimicrobial vulcanizednitrile butadiene or natural rubber-containing article.

Accordingly, this invention encompasses a dimensionally stablevulcanized rubber-containing article, wherein said rubber is selectedfrom the group consisting of nitrile butadiene rubber, natural rubber,and any mixtures thereof, wherein said article exhibits log kill ratesfor Staphylococcus aureus and Klebsiella pneumoniae of at least 1.0 eachafter 24 hours exposure at room temperature. Also, this inventionencompasses a dimensionally stable vulcanized nitrile butadiene ornatural rubber-containing article exhibiting antifungal properties suchthat said article exhibits at least 70% inhibition in accordance withTest Method ISO 486, of Aspergillus niger ATCC 6275 for at least 15 daysat 30° C. and at greater than 90% humidity. Furthermore this inventionencompasses such a vulcanized nitrile butadiene rubber or naturalrubber-containing article comprising at least one silver ion controlrelease additive, such as those selected selected from the groupconsisting of fillers (such as carbon black, calcium carbonate,inorganic salts, organic salts, silica, and mixtures thereof) andplasticizers (oils such as phthalate oils and paraffinic oils).Additionally, this invention encompasses a method of producing avulcanized nitrile butadiene rubber or natural rubber-containing articlecomprising the steps of providing a rubber formulation comprisinguncured rubber, at least one non-sulfur based curing agent, and at leastone silver-based antimicrobial compound, and vulcanizing said rubberformulation at a temperature of at least 150° C. and at least at apressure of 3 bars, wherein said rubber formulation is substantiallyfree from sulfur curing agent and accelerator.

The term “dimensionally stable” is intended to encompass a vulcanizedrubber article that is structurally able to be handled withoutdisintegrating into smaller portions. Thus, the article must exhibitsome degree of structural integrity and, being a rubber, a certaindegree of flexural modulus.

Such a specific antimicrobial vulcanized nitrile butadiene and/ornatural rubber-containing article has not been taught nor fairlysuggested within the rubber industry or prior art. As noted above, theavoidance of sulfur-based curing agents and accelerators to anyappreciable degree thus permits the retention of silver antimicrobialswithin the final product in amounts sufficient to provide long-lastinglog kill rates for Staphylococcus aureus, Klebsiella pneumoniae,Pseudomonas aeruginosa, and Escheria coli, at the very least.Furthermore, due primarily to high costs, non-sulfur curing agents havenot been prevalent within vulcanized rubber formulations and articles.As such, there has been no teaching or fair suggestion of couplingnon-sulfur curing agents (and most preferably peroxide curing agents)with silver-based antimicrobial agents within pre-vulcanized rubberformulations to form effectively antimicrobial vulcanized rubberarticles.

Additionally, generally and preferably, though not necessarily, certainfillers and oils (such as silica, carbon black, stearates as fillers,and phthalate and paraffinic oils) are required to provide both flexuralmodulus and structural integrity to vulcanized rubber articles. Therubber component alone generally does not exhibit proper dimensionalstability without such additives. Surprisingly, the presence of suchadditives also provides the ability to control silver-ion release at thetarget article surface. Without intending to be bound to any specificscientific theory, it appears that such fillers as silica and such oilsas paraffinic oil (as some examples), act in such a way as to drawmoisture into the article which then transports silver ions from withinthe article to the surface. In such a situation, then, the rubberarticle may exhibit enhanced silver release resulting in higher log killrates for certain bacteria due to the presence of larger amounts ofavailable surface silver ions. Other hydrophobic fillers, such aspigments (for example carbon black) and calcium carbonate (as someexamples) appear to work in the opposite manner by keeping water out ofthe target article and thus prevent silver-ion migration to the articlesurface. Thus, the reduction of such silver-ion availability decreasesthe antibacterial efficacy of the rubber article. In effect, then, theactual antibacterial efficacy of the entire rubber article can becontrolled through the presence of certain amounts of such generallyrequired fillers and oils (some hydrophilic antistatic agents alsoappear to act in the same manner as silica as well). As a result, thenecessary filler and/or oil constituents required to provide dimensionalresiliency and/or flexural modulus (and thus actual usefulness) of thefinished article serve a dual purpose heretofore unrecognized within therubber industry. Rubber articles can be produced with specific end-usesin mind depending upon the duration of antimicrobial activity desiredthrough the addition of specific amounts of such additives. Again, sucha targeted duration antimicrobial vulcanized article and the benefitsthereof have heretofore been unknown and unrecognized within the rubberindustry. These rubber components are thus hereinafter referred to as“silver ion release control additives”.

The term nitrile butadiene rubber is intended to cover any standardrubber which possessing at least some nitrile and butadiene moietiestherein and which must be vulcanized to provide a dimensionally stablerubber article. Natural rubber is intended to include all rubbersharvested from a natural source (e.g., rubber trees) as well as suchrubber compositions which originate from such a natural source but whichmay be processed in some manner before incorporation within the targetarticle. The specific types are listed below and have been utilized formany years and are generally well known and taught throughout the priorart. Such inventive rubber formulations should also possess a chemicalplasticizer which aids in the breakdown period of the elastomer duringcompounding and processing (and provides flexural modulus properties tothe finished article) as well as fillers required for reinforcement(e.g. calcium carbonate, carbon black, silica, and clays). Optionally,to form a blown (foam or sponge) rubber type, a blowing agent may beadded to the inventive formulation.

The rubber component or components of the inventive raw rubberformulation is therefore selected from the group consisting of nitrilerubber [such as acrylonitrile-butadiene (NBR)], and other types,including, as noted above, natural rubber, and any mixtures of suchrubber types. Other types of rubber may also be mixed with the basebutadiene types in order to provide different strengths, flexibilities,or other properties (such as ozone resistance with EPDM additions). Suchother rubber components may then be, without limitation, chloroprene,ethylene propylene diene modified rubber (EPDM), polyurethane rubber,butyl rubbers, neoprene, isoprene, halobutyl rubbers, fluoroelastomers,epichlorohydrin rubber, polyacrylate rubber, and chlorinatedpolyethylene rubber, as long as the majority of the mixture is comprisedof butadiene rubber. Modified rubbers which are potentially useful,though more expensive, include hydrogenated SBR, hydrogenated NBR, andcarboxylated NBR and the like, and may make up the majority as these arebutadiene-based. Although the presence of silicone-rubber is discouragedwithin the inventive formulation, there remains the possibility ofadding certain low amounts of such specific unvulcanized rubbercomponents without adversely affecting the overall antimicrobial rubberformulation itself. Thus, up to 25% by total weight of the formulationmay be silicone-rubber; however, the vast majority of the rubberformulation must be non-silicone rubber. Thus, with a total number ofparts between about 300 and 2,000 parts throughout the target vulcanizedrubber article, the rubber constitutes from about 25 to about 70% of thepercentage by parts of the entire article. The remainder comprisesadditives such as fillers, oils, curing agents, the desiredantimicrobial agents, optional blowing agents, and the like (asdiscussed more thoroughly below).

Furthermore, the non-silicone rubber portion must not possess anappreciable amount of sulfur-based curing agent or residue (in thefinished article) and thus must be vulcanized through curing withprimarily non-sulfur-based compounds (such as peroxides and metaloxides, for example). The rubber component is present in amount of fromabout 10 to about 1,000 parts of the entire composition, more preferablyfrom about 50 to about 500 parts, and most preferably from about 100 toabout 200 parts.

The antimicrobial agent of the inventive raw rubber formulation may beof any standard silver-based compounds. Such compounds, in contrast withorganic types, such as triclosan, for example, do not exhibit lowthermal stability and thus remain within the target matrix or substrateat different temperatures. Thus, such an antimicrobial is more easilycontrolled, as discussed above, for surface release as desired. Suchagents include, without limitation, silver salts, silver oxides,elemental silver, and, most preferably ion exchange, glass, and/orzeolite compounds. Of even greater preference are silver-based ionexchange compounds for this purpose due to the low levels ofdiscoloration and enhanced durability in the final product provided bysuch compounds, the efficacy provided to the final formulation with sucha compound, and the ease of manufacture permitted with such specificcompounds. Thus, the antimicrobial agent of this invention may be anytype which imparts the desired log kill rates as previously discussed toStaphylococcus aureus, Klebsiella pneumoniae, Escherichia coli, andPseudomonas aeruginosa, as merely representative organisms. Furthermore,such antimicrobial compounds must be able to withstand elevatedprocessing temperatures for successful incorporation within the targetnon-sulfur (peroxide, for example) cured rubber formulations. Again,such antimicrobial agents comprise, preferably, silver-containing ionexchange, glass, and/or zeolite compounds. Most preferably, such acompound is a silver-based ion-exchange compound and particularly doesnot include any added organic bactericide compounds (thereby notpermitting a release of volatile organic compounds into the atmosphereduring processing at high temperatures, etc.). The preferredsilver-based ion exchange material is an antimicrobial silver zirconiumphosphate available from Milliken & Company, under the trade nameALPHASAN®. Such compounds are available in different silver ionconcentrations as well as mixtures with zinc oxide. Thus, differentcompounds of from about 0.01 to 10% of silver ion concentration,preferably from about 3 to about 8%,, and most preferably amounts ofabout 3, 3.8, and 10% by total amount of components (e.g. of the totalamount of silver ions and zirconium phosphate) are possible. Otherpotentially preferred silver-containing solid inorganic antimicrobialsin this invention are silver-substituted zeolite available from Sinanenunder the tradename ZEOMIC®, or a silver-substituted glass availablefrom Ishizuka Glass under the tradename IONPURE®, which may be utilizedeither in addition to or as a substitute for the preferred species.Other possible compounds, again without limitation, are silver-basedmaterials such as MICROFREE®, available from DuPont, as well as JMAC®,available from Johnson Mathey.

Generally, such an antimicrobial compound is added to a rubberformulation in an amount of from about 0.1 to 10% by total weight of theparticular total rubber formulation; preferably from about 0.1 to about5%; more preferably from about 0.1 to about 2%; and most preferably fromabout 0.2 to about 2.0%.

Furthermore, with regard to silver-based inorganic antimicrobialmaterials, these particular antimicrobial rubber articles are shown tobe particularly suitable for the desired high levels of efficacy anddurability required of such articles. It has been found that certainsilver-based ion exchange compounds, such as ALPHASAN® brandantimicrobials available from Milliken & Company, (U.S. Pat. Nos.5926238, 5441717, 5698229 to Toagosei Chemical Industry Inc.), exhibitimpressive bio-efficacy. After a period of time, alternativeantimicrobial compounds (e.g. triclosan, microchek, OBPA, Zn-omadine)initially suffer from decomposition under the high processingtemperatures, followed by depletion of the biocide through leaching intothe surrounding environment and finally through depleted bactericidalactivity. However, silver-containing ion exchange, glass, and/or zeolitecompounds do not suffer from these shortcomings. Such antimicrobialagents exhibit high temperature stability (>1000° C.), do not leach intothe environment and provide substantial amounts of the oligodynamicsilver ion to provide for the desired extensive durability.

The inventive antimicrobial articles should exhibit an acceptable logkill rate after 24 hours in accordance with the AATCC Draft Test Methodentitled “Assessment of Antimicrobial Properties on Hydrophobic Textilesand Solid Substrates” as well as in accordance with Japanese Test MethodJIS 2 2801. Such an acceptable level log kill rate is tested forStaphylococcus aureus or Klebsiella pneumoniae of at least 0.1 increaseover baseline. Alternatively, an acceptable level will exist if the logkill rate is greater than the log kill rate for non-treated. (i.e., nosolid inorganic antimicrobial added) rubber articles (such as about 0.5log kill rate increase over control, antimicrobial-free vulcanizednitrile and/or natural rubber). Preferably these log kill rate baselineincreases are at least 0.3 and 0.3, respectively for S. aureus and K.pneumoniae; more preferably these log kill rates are 0.5 and 0.5,respectively; and most preferably these are 1.0 and 1.0, respectively.Of course, the high end of such log kill rates are much higher than thebaseline, on the magnitude of 5.0 (99.999% kill rate). Any rate inbetween is thus, of course, acceptable as well. However, log kill rateswhich are negative in number are also acceptable for this invention aslong as such measurements are better than that recorded for correlatednon-treated rubber articles. In such an instance, the antimicrobialmaterial present within the rubber article at least exhibits a hindranceto microbe growth. Furthermore, such rubber articles should exhibit logkill rates of the same degree for other types of bacteria, such as,Psedumonas aeruginosa and Eschericia coli.

Of great surprise within this invention is the ability for the finishedinventive articles to provide antifungal benefits as well asantibacterial characteristics. Such versatility is rare amongantibacterial compounds; however, without intending to be limited to anyparticular scientific theory, it appears that the silver ions, andparticularly the silver ions present at the article surface in greatabundance, provide excellent antifungal properties. Thus, anotherembodiment of this inventive rubber formulation should provide fungalkill durability of at least 15 sequential days for such organisms asAspergillus niger and mixtures of fungi including Aspergillus niger ATCC6275, Paecilomyces variotii ATCC 18502, Trichoderma virens ATCC 9645. Inorder to provide a greater array of potential antifungal benefits, othercompounds may be incorporated within the target pre-vulcanized rubberformulation (and subsequent article), such as zinc oxide, as oneexample.

Of great importance to the effectiveness of the inventive articles interms of antimicrobial and antifungal activity is the omission ofdeleterious amounts of sulfur-based curing agents and accelerators fromthe rubber article. As noted above, sulfur reacts with the preferredsilver-based antimicrobials and irreversibly binds the silver ions (assilver sulfides, for example) within the rubber composition and/orarticle itself. As such, the resultant silver sulfides, etc., areineffective as antimicrobial agents and their presence thus renders thefinal product antimicrobially inactive. Thus, it has been necessary toproduce a vulcanized rubber article lacking any appreciable amount ofsulfur curing agents and accelerators therein. It should be appreciatedthat the term “appreciable amount” permits a small amount to be present.It has been found that, as a molar ratio, a 1:1 ratio (and above)between sulfur molar presence and silver molar presence results in aclear loss of antimicrobial activity within the desired ultimatevulcanized article. However, greater molar amounts of silver in relationto sulfur provide at least some antimicrobial properties to the desiredarticle. A molar ratio range of from 0.25:1 to about 0.000000001:1 ofsulfur to silver ions is thus at least acceptable. The primary curingagent, however, must be of non-sulfur nature (and is preferably, thoughnot necessarily) a peroxide-based compound in order to provide thedesired antimicrobial activity for the subject rubber. Although peroxidecuring agents have been utilized for vulcanization of rubber previously,such a different type of curing agent is not widely utilized as asuitable vulcanization catalyst for rubber for a number of reasons.Foremost, such curing agents are much more costly than standardsulfur-based agents and thus the utilization of such peroxides, and thelike, as a replacement for the sulfur-based compounds have been ratherlimited to mostly silicone-based rubbers or, at the very least,non-antibacterial rubber articles. However, due to the problemsassociated with antimicrobial activity when such compounds are reactedwith sulfur-based curing agents, alternatives to such sulfur-based curedarticles was to permit utilization of such effective antimicrobialcompounds within raw and vulcanized rubber for long-term high log killrate effects. Thus, although non-sulfur-based compounds are not readilyutilized within the non-silicone industry as vulcanization curingagents, utilization of such curing agents was necessary to provide aneffective, ultimate antimicrobial vulcanized rubber article.

Surprisingly, it has now been found that the inventive rubber articleslisted above are available without such sulfur-based curing agents inany appreciable amounts; most importantly, with the introduction ofcertain additives, the structural integrity and/or flexural modulus ofthe rubber formulation is improved to an acceptable level and theefficacy of the antimicrobial components are can be controlledsimultaneously.

Thus, the curing agent present within the raw rubber formulation to bevulcanized to from the inventive article must be at least a majority,and preferably at least about 75% by weight of a non-sulfur-based curingagent. As discussed above, traditional sulfur and sulfur-based catalystswill not work with the inventive antimicrobial formulations due tochemical reactions between the sulfur atoms and and the biocidal Ag+ion.However, non-sulfur-based catalysts, such as, for example, and withoutintending to being limited to peroxides, certain compounds provideeffective curing for the inventive raw rubber formulations, such asorganic peroxides, including dicumyl peroxide,2,5-bis(t-butylperoxy)-2,5-dimethylhexane, di-(t-butyl-peroxy-isopropyl)benzene, di-(t-butyl-peroxy-trimethyl)-cyclohexane, and the like, andinorganic peroxides and oxides, including zinc oxide, and the like. Sucha curing agent should be present in amount of from about 0.5 to about100 parts per hundred parts of rubber (pphr); more preferably from about1 to about 50 pphr; and most preferably from about 2 to about 10 pphr,all either as one curing agent alone, or as the combination of anynumber of different types.

Other additives to the rubber formulation include any of theaforementioned silver ion release control additives, accelerators,accelerator activators, antidegradants, softeners, abrasives, colorants,flame retardants, homogenizing agents, internal lubricants, anddeodorants. Such components should be present, if at all, in rather lowamounts, of from about 0. 1 to about 10 pphr.

It has further been unexpectedly determined that a substantial increasein the antimicrobial and antifungal efficacy is provided upon washingthe finished inventive article. Abrading the surface of such an articlealso permits increases in such characteristics; however, industriallaundering of certain rubber products (mats, and the like) can beimproved in antimicrobial, etc., efficacy through a simple washing. Infact, such an increase steadily improves with greater numbers ofconsistent washes such that it has been found that a rubber article asfirst vulcanized exhibits lower overall antimicrobial and antifungalactivity than one that has been washed one, two, three, and up to atleast 20 times (with a standard detergent such as a mixture of anionicsurfactants, in a standard industrial rotary washing machine). Such asurprising benefit thus permits utilization of such rubber articles asfloor coverings (mats, as one example, such as those with carpetedportions or those which are rubber alone; particularly foamed rubbermats for antifatigue properties and reduced specific gravity so as toreduce the chances of machinery damage during such industrial rotarylaunderings and dryings), and other articles which can be easily washedwithin standard laundry machines.

Furthermore, as alluded to above, friction with the subject rubberarticle surface can remove very slight layers of rubber from the articlesurface thereby permitting “fresh” silver-comprising crystallites to thesurface to act as desired in their antibacterial and/or antifungalcapacities. Basically, then, the inventive article produced from theinventive raw rubber formulation exhibits an even dispersion ofantimicrobial particles throughout the entire rubber article. Such aneven dispersion of the biocide throughout the rubber article thusprovides a reservoir of fresh crystallites containing the biocidalmetallic ion. As layers of the rubber are worn and abraded away,antimicrobial particles containing untapped silver ions becomeavailable.

The preferred peroxide cured rubber articles of this inventioncontaining the antimicrobial agent can be processed into rubber articleswhich exhibit excellent antimicrobial qualities as well as antimicrobialefficiency throughout the rubber article's lifetime. Examples of othersuch rubber articles encompassed within this invention include, but arenot limited to hard rubber mats, static dissipative rubber mats,anti-fatigue rubber mats, rubber mats which include a face fiber, rubberlink mats, rubber gaskets, rubber medical goods, rubber gloves, rubbermedical devices, rubber conveyor belts, rubber belts and rubber wheelsused in food processing, rubber clothing, rubber shoes, rubber boots,rubber tubing, and rubber automotive fuel hoses. Such inventiveformulations may also be incorporated into a multilayered rubber articlein which the antimicrobial agent can be incorporated into any surfacelayer and still provide the desired antimicrobial efficiency.

Of particular interest is the formation of multilayered rubber articleswherein at least one of such rubber layer exhibits the desiredantimicrobial activity and thus is made from an inventive raw rubberformulation. Such layered articles may be adhered together throughco-vulcanization, gluing, and the like. Furthermore, layers of othertypes of materials may be placed being rubber layers as well to provide,as one non-limiting property, better structural stability to the desiredmultilayered article.

The non-limiting preferred embodiments of these rubber formulations andarticles are discussed in greater detail below.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Inventive Raw Rubber Formulations

(Inventive) NBR Base Formulation 1

Component Amount Acrylonitrile butadiene Rubber (from Zeon Chemicals)100 parts   FEF N550 50 pphr  Stearic acid 1 pphr Microcrystalline wax 2pphr Polyethylene glycol 5 pphr Silica 40 pphr  Zinc oxide 5 pphrCalcium carbonate 20 pphr  di-octyl-phthalate 3 pphrdi-(tert-butyl-peroxy-isopropyl)benzene 4 pphrdi-(tert-butyl-peroxy-trimethyl)-cyclohexane 4 pphr Antimicrobial asnoted

(Inventive) NBR Base Formulation 2

Component Amount Acrylonitrile butadiene Rubber 100 parts  Pentaerythritol tetrastearate (processing aid) 1 pphr Microcrystallinewax 1 pphr FEF N550 50 pphr  Silica 20 pphr  Stearic acid 1 pphr Zincoxide 5 pphr Calcium carbonate 20 pphr  di-octyl-phthalate 20 pphr Azodicarbonic diamide (blowing agent) 3 pphr Oxy-bis(benzenesulfonylhydrazide) (blowing agent) 3 pphrdi-(tert-butyl-peroxy-isopropyl)benzene 3 pphrdi-(tert-butyl-peroxy-trimethyl)-cyclohexane 3 pphr Antimicrobial asnoted

(Inventive) Natural Rubber Base Formulation

Component Amount Natural Rubber 100 parts   Pentaerythritoltetrastearate (processing aid) 2 pphr Polyethylene glycol 5 pphr Silica40 pphr  Calcium oxide (dessicant) 10 pphr  Titanium dioxide 5 pphr Zincoxide 3 pphr Calcium carbonate 20 pphr  Paraffinic oil 20 pphr Ethyleneglycol dimethacrylate 2.5 pphr  di-(tert-butyl-peroxy-isopropyl)benzene 1 pphrdi-(tert-butyl-peroxy-trimethyl)-cyclohexane 1 pphr Antimicrobial asnoted

Specific samples with different silver ion-exchange zirconium phosphatesalts (available from Milliken & Company under the tradename ALPHASAN®)were made from these base formulations and are listed below. Thedifferent biocides presented throughout are labeled in accordance withthe following table:

TABLE Silver Biocide ion concentration Other components (% by weight) A3.8% Ag_(x)Na_(y)H_(z)Zr₂(PO₄)₃ where x + y + z = 1 B 10.0% Ag_(x)Na_(y)H_(z)Zr₂(PO₄)₃ where x + y + z = 1 C 3.0% Zinc oxide (70%)(with 30% biocide B)

The compounding of ingredients within each formulation can be carriedout in an open mill, an internal mixer, or an extruder where intensivemixing within the polymer matrix of each component will take place.During the mixing operation, the control of temperature rise, due tohigh shear incorporation of the ingredients, is crucial to ensure thatpre-vulcanization (scorch) does not take place during processing.Generally, a maximum temperature of 120° C. is reached on single stage(pass) mixing through an internal mixer. The compounds can be furtherprocessed after mixing into specific forms to allow adequatepresentation for manufacturing into products. This could be calendering,extrusion, granulation/pelletization, strip form, fabrication andperforming into specific shaped blanks.

The vulcanization of the compounds can be in the form of molding(compression, transfer, injection), continuous extrusion (LCM, UHF[wherepermissible], autoclave and hot air), and coatings. The vulcanization(cure) temperatures can range from 150° C. to 250° C. In this specificsituation, the rubber articles were calendared into rough mat structuresand then subjected to vulcanization under high temperature and pressure.

Testing of Vulcanized Rubber Articles

The following Tables list the antibacterial and antifungal activity ofthese inventive and comparative samples. The antimicrobial testsfollowed were AATCC Draft Test Method “Assessment of AntimicrobialProperties on Hydrophobic Textiles and Solid Substrates” and JapaneseMethod JIS2 2801 for Staphylococcus aureus and the antifungal testsfollowed were ISO 486 for Aspergillus niger and a mixture of fungiincluding Aspergillus niger, Paecilomyces variotii, and Trichodermavirens. Further tests, such as silver-ion extraction within an aqueoussalt solution and testing the liquor with any extracted silver ionstherein under inductively coupled plasma, were followed to analyze theeffectiveness of certain additives (silica, carbon black, phthalateoils) in relation to antimicrobial efficacy as well. Lastly, industrialwashing of such mats was undertaken in an effort to determine theimprovements (if any) of the antimicrobial activity of certain samples.

EXPERIMENTAL TABLE 1 Antimicrobial Performance of rubber formulationsfor Staphylococcus aureus and Klebsiella pneumoniae log kill reductionvs. internal control Sample ID biocide ID S. aureus K. pneumoniae NBRFormulation 1 no biocide x 0 NBR Formulation 2 1% biocide A x 3.87 NBRFormulation 1 1% biocide C x 4.01 Natural Rubber Formula- no biocide x−1.98 tion Natural Rubber Formula- 1% biocide A x −1.62 tion NaturalRubber Formula- 1% biocide C x −1.19 tion

Thus, the inventive formulations provided inventive vulcanized rubberarticles that exhibited improved antimicrobial activity over the sameformulations without any antimicrobial compounds present.

EXPERIMENTAL TABLE 2 Antifungal Performance of Inventive Rubber ArticlesFungal Growth After Example Test Fungi 0 5 10 15 20 NBR Formulation 1Aspergillus niger 0 0 0 0 0 NBR Formulation 1 mixture* 0 0 1 2 3

Efficacy against fungi was assessed using ISO Method 846 againstAspergillus niger ATCC 6275. The mixture of fungi includes Aspergillusniger ATCC 6275, Paecilomyces variotii ATCC 18502, Trichoderma virensATCC 9645. Samples were placed on Potato Dextrose Agar (PDA) andinoculated with 10 droplets (10 ul each, 100 ul total) of 10E5 fungalspores/ml in a synthetic nutrient medium followed by incubation for 7-20days at 30° C. and >90% relative humidity. Efficacy was measured byvisual observation of the samples.

Observation Rating None 0 Traces of Growth (less than 10%) 1 LightGrowth (10-30%) 2 Medium Growth (30-60%) 3 Heavy Growth (60% to completecoverage) 4

EXPERIMENTAL TABLE 3 Antimicrobial Activity of inventive NBR formulationarticles after industrial washing. wash K. S. Sample ID biocide cyclespneumoniae aureus NBR Formulation 1 no biocide 0 wash −0.33 −0.10 NBRFormulation 1 no biocide 1 wash −0.72 −0.14 NBR Formulation 1 no biocide20 wash  −0.37 −0.09 NBR Formulation 1 1% biocide A 0 wash −0.01 0.16NBR Formulation 1 1% biocide A 1 wash 3.67 2.17 NBR Formulation 1 1%biocide A 20 wash  4.15 2.08 NBR Formulation 2 1.5% biocide A 0 wash−0.48 x NBR Formulation 2 1.5% biocide A 1 wash −0.56 x NBR Formulation2 1.5% biocide A 20 wash  3.54 3.77

Thus, the inventive articles not only retained the antimicrobial andthus exhibited antimicrobial activity after washing, such activityactually and unexpectedly improved as the number of washes increased.This indicates that the amount of effective antimicrobial silver ionsavailable at the target article surface increased as the number ofwashings increased. For such a measurement, the target article wasimmersed in an aqueous salt extraction solution (sodium chloride) for 24hours; the extract was then analyzed by inductively coupled plasmameasurements for a measurement of available silver removed from thearticle surface. The results are as follows:

EXPERIMENTAL TABLE 4 Available Surface Silver Ion Amounts Of InventiveNBR Mats After Industrial Washing. ppb of bio- Sample ID biocide washcycles available Ag⁺/cm² NBR Formulation 1 no biocide 0 wash 0 NBRFormulation 1 no biocide 1 wash 0 NBR Formulation 1 no biocide 20 wash 0 NBR Formulation 1 1% biocide A 0 wash 0.02 NBR Formulation 1 1%biocide A 1 wash 0.7 NBR Formulation 1 1% biocide A 20 wash  1.68 NBRFormulation 2 1.5% biocide A 0 wash 0.07 NBR Formulation 2 1.5% biocideA 1 wash 0.23 NBR Formulation 2 1.5% biocide A 20 wash  1.87

Thus, surprisingly, the amount of available silver ions increaseddramatically not just from the mat's finished state, but also from aftera single wash and up to (and beyond) twenty standard washes. Such anunexpected benefit thus provides the user with an antimicrobial matstructure that increases its antimicrobial efficacy during use.

Having described the invention in detail it is obvious that one skilledin the art will be able to make variations and modifications theretowithout departing from the scope of the present invention. Accordingly,the scope of the present invention should be determined only by theclaims appended hereto.

What is claimed is:
 1. A dimensionally stable vulcanized rubber articlecomprising at least a majority of a rubber constituent selected from thegroup consisting of nitrile butadiene rubber, natural rubber, and anymixtures thereof, and at least one silver-based antimicrobial agent,wherein said rubber article exhibits log kill rates in accordance withthe AATCC Draft Method entitled “Assessment of Antimicrobial Propertieson Hydrophobic Textiles and Solid Substrates” for Staphylococcus aureusand Klebsiella pneumoniae of at least 1.0 each after 24 hours exposureat room temperature, and wherein said article optionally comprises atleast one silver ion release control additive, and at least oneantifungal additive other than said silver-based antimicrobial compound.2. The rubber article of claim 1 wherein said article exhibits log killrates for Staphylococcus aureus and Klebsiella pneumoniae of at least2.0 each after 24 hours exposure at room temperature.
 3. The rubberarticle of claim 1 wherein said silver-based antimicrobial compound isselected from the group consisting of elemental silver, silver oxides,silver salts, silver ion exchange compounds, silver zeolites, silverglasses, and any mixtures thereof.
 4. The rubber article of claim 2wherein said silver-based antimicrobial compound is selected from thegroup consisting of elemental silver, silver oxides, silver salts,silver ion exchange compounds, silver zeolites, silver glasses, and anymixtures thereof.
 5. The rubber article of claim 1 wherein said at leastone silver ion control release additive is present.
 6. The rubberarticle of claim 1 wherein said antifungal additive other than saidsilver-based antimicrobial compound is present.
 7. The rubber article ofclaim 5 where said at least one silver ion control release additive isselected from the group consisting of fillers, oils, pigments, salts,antistatic agents, and any mixtures thereof.
 8. The rubber article ofclaim 7 wherein said at least one silver ion control release additive isa hydrophilic filler selected from the group consisting of silica,stearates, and any mixtures thereof.
 9. The rubber article of claim 8further comprising at least one hydrophilic oil selected from the groupconsisting of paraffinic oil, phthalate oil, and any mixtures thereof.10. A dimensionally stable vulcanized rubber-containing articlecomprising at least a majority of a rubber constituent selected from thegroup consisting of nitrile butadiene rubber, natural rubber, and anymixtures thereof, and at least one silver-based antimicrobial agent,wherein said article exhibits antifungal properties such that saidarticle exhibits at least 70% inhibition in accordance with Test MethodISO 486 of Aspergillus niger ATCC 6275 for at least 15 days at 30° C.and at greater than 90% humidity, wherein said article optionallycomprises at least one silver ion release control additive, and at leastone antifungal additive other than said silver-based antimicrobialcompound.
 11. The rubber article of claim 10 wherein said silver-basedantimicrobial compound is selected from the group consisting ofelemental silver, silver oxides, silver salts, silver ion exchangecompounds, silver zeolites, silver glasses, and any mixtures thereof.12. The rubber article of claim 10 wherein said at least one silver ioncontrol release additive is present.
 13. The rubber article of claim 10wherein said antifungal additive other than said silver-basedantimicrobial compound is present.
 14. The rubber article of claim 12where said at least one silver ion control release additive is selectedfrom the group consisting of fillers, oils, pigments, salts, antistaticagents, and any mixtures thereof.
 15. The rubber article of claim 14wherein said at least one silver ion control release additive is ahydrophilic filler selected from the group consisting of silica,stearates, and any mixtures thereof.
 16. The rubber article of claim 15further comprising at least one hydrophilic oil selected from the groupconsisting of paraffinic oil, phthalate oil, and any mixtures thereof.17. The rubber article of claim 1 wherein said article is a matstructure.
 18. The rubber article of claim 1 wherein said article is amat structure.
 19. An antimicrobial vulcanized rubber article exhibitingan increase in log kill rates in accordance with the AATCC Draft Methodentitled “Assessment of Antimicrobial Properties on Hydrophobic Textilesand Solid Substrates” for Staphylococcus aureus and Klebsiellapneumoniae when measured first for antimicrobial efficacy after initialarticle production as compared with subsequent measurement forantimicrobial efficacy after said article is exposed to one standardlaundering within a standard industrial rotary washing machine.
 20. Theantimicrobial vulcanized rubber article of claim 19 exhibiting anincrease in log kill rates in accordance with the AATCC Draft Methodentitled “Assessment of Antimicrobial Properties on Hydrophobic Textilesand Solid Substrates” for Staphylococcus aureus and Klebsiellapneumoniae when measured first for antimicrobial efficacy after said onestandard laundering as compared with subsequent measurement forantimicrobial efficacy after said article is exposed to twenty standardlaunderings.